1. Field of the Invention
The present invention relates to a semiconductor device which has a buffer layer on a substrate and which is produced by use of an epitaxial crystal growth method and to a method of manufacturing the device.
2. Description of the Prior Art
Conventinally, there has been reported that by providing a superlattice buffer layer of GaAs-AlGaAs on a substrate, the quality of a group III-V semiconductor device formed with atoms of group or family III and atoms of family V, and in particular, the quality of a laser of such a semiconductor device can be increased ("MBE growth of exteremely high-quality GaAs-AlGaAs GRIN-SCH lasers with a superlattice buffer layer", T. Fujii et al., j. Vac. Sci. Technol. B3 (2), Mar/Apr 1985, pp 776-778). According to this report, when AlGaAs is grown on a GaAs substrate, or when an optical semiconductor device or an electronic device is grown thereon, a GaAs layer and an AlGaAs layer each having a layer thickness of about 15 nanometers are alernately formed several times in the AlGaAs layer so as to constitute a superlattice buffer layer, which traps or captures residual gases in the periphery of the substrate; in consequence, impurities are prevented from entering actual device layers formed thereon. It has been reported that this results in an improvement in the strength of the light emitted from a single quantum well as an actual device configured thereon, namely, 40 times the original strength.
However, the superlattice buffer layer is also not efficient as a solution against distortion or the like. For example, in a case where a light emitting element is assumed as a semiconductor device to be manufactured on a substrate so as to reduce the wavelength of the emitted wave to a short wave range, the ratio of the component of Al becomes large in the AlGaAs layer. In this situation, the difference between the coefficient of thermal expansion of the substrate and that of the AlGaAs layer becomes great, which leads to a fear that the distortion in the grown layer is increased and the reliability of the device is lowered. The superlattice buffer layer in the report is not effective for the problems above.